The invention relates to copying machines and, more particularly, to sheet feeding devices for feeding sheets singly from a stack to a utilization device.
Both bottom level feeders, which feed single sheets from the bottom of a stack, and top level feeders, which feed the top sheet from a stack, as well known and are used extensively with copying machines. The feeding of single sheets from the bottom of a stack of sheets, while generally more difficult, is desirable in that additional sheets may be added to the stack during the feeding operation and the document order is preserved.
Many prior art sheet feeding devices utilize a flexible feed belt to draw the exterior or outermost sheet from a stack, and a substantially rigid retard member to prevent sheets adjacent to the exterior sheet from being simultaneously completely withdrawn and expelled from the stack. Examples of such feeders are Stange U.S. Pat. No. 3,768,803, which shows a top level feeder, and Stange U.S. Pat. No. 4,014,537, which shows a bottom level feeder. Both feeders utilize flexible drive belts and devices which act with the belt to allow only a single sheet to be fed at a time. The bottom level feeder utilizes air flotation to assist in sheet separation.
Many belt-type feed devices utilize retard pads which are made of a relatively high friction material and are positioned to engage the belt during feeder operation to form a pinch point which allows only a single sheet to pass through. An example of a retard pad is shown in Perun et al. U.S. Pat. No. 4,192,497. Other prior art patents disclosing bottom level sheet feeders are David U.S. Pat. No. 3,831,928, which discloses a rigidly supported drive belt and a rigid, pivotable, retard device; Strobel U.S. Pat. No. 3,934,869, which shows a flexible drive belt and rigid retard plus a flow of pressurized air against the bottom sheet to reduce friction between the bottom sheet and the sheet immediately thereabove, and Hamlin et al. U.S. Pat. No. 4,166,614, which shows the desirability of continuously applying an effective normal force to the top of widely variable size stacks for a bottom sheet feeder having a flexible drive belt and a rigid retard.
Sheet feed devices of the type previously described were often difficult to make reliable. Should a large number of sheets enter the pinch point formed by the belt and retard pad, they became shingled by the shearing force exerted by the moving belt, and forced the belt and retard apart. This gap became so large that sheets spaced from the retard pad would be conveyed past the pinch point along with the sheet contacting the belt, a condition known as "multifeed." Multifeeds may be reduced in frequency of occurrence by increasing the force exerted between the belt and pad. However, increasing the pinching force at the pinch point may result in a condition known as "misfeed," in which no sheet is passed through the pinch point; or it may damage the sheet fed.
Accordingly, there is a need for a sheet feeding device which feeds sheets singly from a stack with a minimum amount of multifeeds or misfeeds. There is also a need for a sheet feeding device which maintains a pinch point in response to contact with a large number of sheets such that multifeeds are prevented, and yet does not damage sheets passing through the pinch point.